A reduction in weight has been desired for an automotive heat exchanger made of aluminum in order to achieve a reduction in fuel consumption of an automotive engine and a reduction in cost, and a reduction in thickness of a material (e.g., tube) for producing a heat exchanger has been desired. However, since leakage of a refrigerant due to pitting corrosion of the aluminum alloy member may occur within a shorter period when the thickness of the material is reduced, it is important to provide the material with corrosion resistance while reducing the thickness of the material.
For example, a condenser used for an automotive heat exchanger is produced using a multi-port extruded tube having a flat cross-sectional shape as a tube that forms a refrigerant passage. When KZnF3 is applied to the outer circumferential surface of the tube, and the tube is brazed, KAlF4 is produced by the substitution reaction between Zn and Al, and removes an oxide film formed on the surface of the aluminum alloy. On the other hand, Zn produced by the substitution reaction forms a Zn diffusion layer on the surface of the aluminum alloy member, and improves corrosion resistance (see Patent Document 1). Specifically, when KZnF3 is applied to the aluminum alloy member, and the aluminum alloy member is brazed, KZnF3 reacts with Al that forms the surface of the aluminum alloy member at about 550° C., and is decomposed into Zn and a potassium fluoroaluminate (e.g., KAlF4 and K2AlF5) (i.e., a noncorrosive flux normally used for brazing). Zn produced by decomposition of KZnF3 diffuses into the surface of the aluminum alloy member, and forms a Zn diffusion layer. On the other hand, the potassium fluoroaluminate removes an oxide film formed on the surface of the aluminum alloy member so that wetting occurs between the filler metal and the aluminum alloy member, and the aluminum alloy member is joined.
The Zn diffusion layer has a natural electrode potential lower than that of the aluminum alloy member that forms the tube, and is preferentially corroded as compared with the aluminum alloy member due to a sacrificial anode effect caused by galvanic action to prevent the tube from undergoing pitting corrosion. Since KZnF3 ensures that the Zn diffusion layer has a uniform Zn concentration as compared with Zn arc spraying, it is possible to suppress contamination of the work environment that occurs when a thermally sprayed powder is scattered around the surface of the tube material, and reduce the application amount.
However, KZnF3 may not normally function during brazing when the oxygen concentration in the brazing furnace is high. In such a case, since an oxide film is not removed, the molten filler metal may not spread, and a fillet may not be formed. When the aluminum alloy member is brazed using KZnF3 in an atmosphere having a high oxygen concentration, Zn and K3AlF6 (having a high melting point) (covered with a thick oxide film) produced from KZnF3 that has reacted with oxygen in the brazing furnace during brazing may remain on the surface of the aluminum alloy member as a residue, whereby the surface of the aluminum alloy member may be discolored, and a deterioration in external appearance may occur.
When KZnF3 is stored in an atmosphere having high humidity, KZnF3 may deteriorate, and not normally function during brazing. In such a case, since an oxide film is not removed, the molten filler metal may not spread, and a fillet may not be formed.
In order to prevent such a situation, it is necessary to store KZnF3 in a storage area in which dehumidification equipment is installed.
In this case, however, since it is necessary to always operate the dehumidification equipment, the electricity cost increases, and frequent maintenance of the dehumidification equipment is required. This results in an increase in production cost.
KZnF3 is easily affected by the flow of the molten filler metal, and may flow together with the filler metal when the filler metal flows toward the fin, and forms a fillet. In this case, the Zn concentration in the surface of the tube between the fillets (for which corrosion resistance is required) decreases, and the Zn concentration in the fillet increases, whereby the fillet is preferentially corroded, and the fin is separated at an early stage.
In order to solve the above problems, a method that utilizes a mixture of KZnF3 and a noncorrosive flux (e.g., KAlF4 or K2AlF5) has been proposed, for example (see Patent Document 2).
Specifically, when the noncorrosive flux that does not easily deteriorate during brazing even in an atmosphere having a high oxygen concentration, and removes an oxide film, is mixed with KZnF3 that reacts with the surface of the aluminum alloy member to remove an oxide film and form a Zn diffusion layer, and the mixture is heated, the flux mixture spreads at a temperature lower than the melting point of the filler metal, and the Zn concentration in the Zn diffusion layer between the fillets becomes uniform.